Supercapacitors (Ultra-capacitors or Electro-chemical Capacitors):
A supercapacitor normally depends on porous carbon electrodes to create a large surface area conducive to the formation of diffuse electric double layer (EDL) charges. The ionic species (cations and anions) in the EDL zones are formed in the electrolyte near an electrode surface when voltage is imposed upon a symmetric supercapacitor (or EDLC). The required ions for this EDL mechanism pre-exist in the liquid electrolyte (randomly distributed in the electrolyte) when the cell is made or in a discharged state.
When the supercapacitor is re-charged, the ions (both cations and anions) already pre-existing in the liquid electrolyte are formed into EDLs near their respective local electrodes. There is no exchange of ions between an anode active material and a cathode active material. The amount of charges that can be stored (capacitance) is dictated solely by the concentrations of cations and anions that pre-exist in the electrolyte. These concentrations are typically very low and are limited by the solubility of a salt in a solvent, resulting in a low energy density.
Since the formation of EDLs does not involve a chemical reaction or an exchange of ions between the two opposite electrodes, the charge or discharge process of an EDL supercapacitor can be very fast, typically in seconds, resulting in a very high power density (more typically 3,000-8,000 W/Kg). Compared with batteries, supercapacitors offer a higher power density, require no maintenance, offer a much higher cycle-life, require a very simple charging circuit, and are generally much safer. Physical, rather than chemical, energy storage is the key reason for their safe operation and extraordinarily high cycle-life.
Another type of supercapacitor is a pseudocapacitor that stores electrical energy by means of reversible faradaic redox reactions on the surface of suitable carbon electrodes. Such an electrode typically is composed of a carbon material (e.g. activated carbon) and a transition metal oxide (or a conjugate polymer), which together form a redox pair. Pseudocapacitance is typically accompanied with an electron charge-transfer between electrolyte and electrode arising from a de-solvated and adsorbed ion whereby only one electron per charge unit participates. This faradaic charge transfer originates from a very fast sequence of reversible redox, intercalation or electrosorption processes. The adsorbed ion has no chemical reaction with the atoms of the electrode (no chemical bonding) since only a charge-transfer occurs.
Despite the positive attributes of supercapacitors, there are several technological barriers to widespread implementation of supercapacitors for various industrial applications. For instance, EDLC supercapacitors possess very low energy densities when compared to batteries (e.g., 5-8 Wh/kg for commercial supercapacitors vs. 20-40 Wh/Kg for the lead acid battery and 50-100 Wh/kg for the NiMH battery). Although a pseudocapacitor can exhibit a higher specific capacitance or energy density relative to the EDLC, the energy density per cell is typically lower than 20 Wh/kg. The conventional pseudocapacitor also suffers from a poor cycle life. Lithium-ion batteries possess a much higher energy density, typically in the range of 150-220 Wh/kg, based on the total cell weight.
Lithium-ion Batteries (LIB):
Although possessing a much higher energy density, lithium-ion batteries deliver a very low power density (typically 100-500 W/Kg), requiring typically hours for re-charge. Conventional lithium-ion batteries also pose some safety concern.
The low power density or long re-charge time of a lithium ion battery is due to the mechanism of shuttling lithium ions between the interior of an anode and the interior of a cathode. During recharge, lithium atoms must diffuse out of a cathode active material (e.g. particles of LiCoO2), migrate through an electrolyte phase, and enters or intercalates into the bulk of an anode active material particles (e.g. graphite particles). Most of these lithium ions have to come all the way from the cathode side by diffusing out of the bulk of a cathode active particle, through the pores of a solid separator (pores being filled with a liquid electrolyte), and into the bulk of a graphite particle at the anode.
During discharge, lithium ions diffuse out of the anode active material (e.g. de-intercalate out of graphite particles 10 μm in diameter), migrate through the liquid electrolyte phase, and then diffuse into the bulk of complex cathode crystals (e.g. intercalate into particles lithium cobalt oxide, lithium iron phosphate, or other lithium insertion compound). Because the liquid electrolyte only reaches the external surface (not interior) of a solid particle (e.g. graphite particle), lithium ions swimming in the liquid electrolyte can only migrate (via fast liquid-state diffusion) to the surface of a graphite particle. To penetrate into the bulk of a solid graphite particle would require a slow solid-state diffusion (commonly referred to as “intercalation”) of lithium ions. The diffusion coefficients of lithium in solid particles of lithium metal oxide are relatively low; e.g. typically 10−16-10−8 cm2/sec (more typically 10−14-10−10 cm2/sec), although those of lithium in liquid are approximately 10−6 cm2/sec.
As such, these intercalation or solid-state diffusion processes require a long time to accomplish because solid-state diffusion (or diffusion inside a solid) is difficult and slow. This is why, for instance, the current lithium-ion battery for plug-in hybrid vehicles requires 2-7 hours of recharge time, as opposed to just seconds for supercapacitors. The above discussion suggests that an energy storage device that is capable of storing as much energy as in a battery and yet can be fully recharged in one or two minutes like a supercapacitor would be considered a revolutionary advancement in energy storage technology.
Lithium Ion Capacitors (LIC):
A hybrid energy storage device that is developed for the purpose of combining some features of an EDL supercapacitor (or symmetric supercapacitor) and those of a lithium-ion battery (LIB) is a lithium-ion capacitor (LIC). A LIC contains a lithium intercalation compound (e.g., graphite particles) as an anode and an EDL capacitor-type cathode (e.g. activated carbon, AC). In a commonly used LIC, LiPF6 is used as an electrolyte salt, which is dissolved in a solvent, such as propylene carbonate. When the LIC is in a charged state, lithium ions are retained in the interior of the lithium intercalation compound anode (i.e. micron-scaled graphite particles) and their counter-ions (e.g. negatively charged PF6−) are disposed near activated carbon surfaces.
When the LIC is discharged, lithium ions migrate out from the interior of graphite particles (a slow solid-state diffusion process) to enter the electrolyte phase and, concurrently, the counter-ions PF6− are also released from the EDL zone, moving further away from AC surfaces into the bulk of the electrolyte. In other words, both the cations (Li+ ions) and the anions (PF6−) are randomly disposed in the liquid electrolyte, not associated with any electrode. This implies that the amounts of both the cations and the anions that dictate the specific capacitance of a LIC are essentially limited by the solubility limit of the lithium salt in a solvent (i.e. limited by the amount of LiPF6 that can be dissolved in the solvent) and the surface area of activated carbon in the cathode. Therefore, the energy density of LICs (a maximum of 14 Wh/kg) is not much higher than that (6 Wh/kg) of an EDLC (symmetric supercapacitor), and remains an order of magnitude lower than that (most typically 150-220 Wh/kg) of a LIB.
Furthermore, due to the need to undergo de-intercalation and intercalation at the anode, the power density of a LIC is not high (typically<12 kW/kg, which is comparable to or only slightly higher than those of an EDLC).
Sodium Ion Batteries and Sodium Compound-Based Supercapacitors
As a totally distinct class of energy storage device, sodium batteries have been considered an attractive alternative to lithium batteries since sodium is abundant and the production of sodium is significantly more environmentally benign compared to the production of lithium. In addition, the high cost of lithium is a major issue and Na batteries potentially can be of significantly lower cost.
There are at least two types of batteries that operate on bouncing sodium ions (Na+) back and forth between an anode and a cathode: the sodium metal battery having Na metal or alloy as the anode active material and the sodium-ion battery having a Na intercalation compound as the anode active material. Sodium ion batteries using a hard carbon-based anode active material (a Na intercalation compound) and a sodium transition metal phosphate as a cathode have been described by several research groups: X. Zhuo, et al. Journal of Power Sources 160 (2006) 698; J. Barker, et al., US Patent Application US2005/0238961, 2005; J. Barker, et al. “Sodium Ion Batteries,” U.S. Pat. No. 7,759,008 (Jul. 20, 2010) and J. F. Whitacre, et al. “Na4Mn9O18 as a positive electrode material for an aqueous electrolyte sodium-ion energy storage device,” Electrochemistry Communications 12 (2010) 463-466.
However, these sodium-based devices exhibit even lower specific energies and rate capabilities than Li-ion batteries. The anode active materials for Na intercalation and the cathode active materials for Na intercalation have lower Na storage capacities as compared with their Li storage capacities. For instance, hard carbon particles are capable of storing Li ions up to 300-360 mAh/g, but the same materials can store Na ions up to 150-250 mAh/g.
Instead of hard carbon or other carbonaceous intercalation compound, sodium metal may be used as the anode active material in a sodium metal cell. However, the use of metallic sodium as the anode active material is normally considered undesirable and dangerous due to the dendrite formation, interface aging, and electrolyte incompatibility problems.
Aqueous electrolyte-based asymmetric or hybrid supercapacitors with a sodium ion intercalation compound (NaMnO2) as the cathode and activated carbon as the anode were investigated by Qu, et al [Q. T. Qu, Y. Shi, S. Tian, Y. H. Chen, Y. P. Wu, R. Holze, Journal of Power Sources, 194 (2009) 1222]. Similar compounds (sodium birnessite, NaxMnO2) were used as the electrode materials of another supercapacitor [L. Athouel, F. Moser, R. Dugas, O. Crosnier, D. Belanger, T. Brousse, Journal of Physical Chemistry C 112 (2008) 7270]. At least the cathode in these supercapacitors involves solid state diffusion (intercalation and de-intercalation) of Na ions in a NaxMnO2 solid. Furthermore, they still exhibit relatively low energy densities.
However, these sodium-based devices exhibit even lower specific energies and rate capabilities than Li-ion batteries. These conventional sodium-ion batteries require sodium ions to diffuse in and out of a sodium intercalation compound at both the anode and the cathode. The required solid-state diffusion processes for sodium ions in a sodium-ion battery are even slower than the Li diffusion processes in a Li-ion battery, leading to excessively low power densities.
The above review of the prior art indicates that a battery has a higher energy density, but is incapable of delivering a high power (high currents or pulsed power) that an EV, HEV, or micro-EV needs for start-stop and accelerating. A battery alone is also not capable of capturing and storing the braking energy of a vehicle. A supercapacitor or LIC can deliver a higher power, but does not store much energy (the stored energy only lasts for a short duration of operating time) and, hence, cannot be a single power source alone to meet the energy/power needs of an EV or HEV. Thus, there is an urgent need for an electrochemical energy storage device that delivers both a high energy density and a high power density.